Electron guns typically comprise an electron emission source or cathode provided with an emission surface, a focusing electrode for directing the emitted electrons to a predetermined spatially confined trajectory, and one or more further electrodes for accelerating and deflecting the emitted electrons toward a target. An electron emission source may be of a thermionic cathode type. A thermionic cathode may be defined as a cathode heated by a heating element, for example an electrical filament, causing the cathode to release electrons with sufficient energy to overcome a work function of the material present on the emission surface. Generally, the focusing electrode is located relatively close to the emission surface of the cathode, and is at the same potential as the cathode. The shape of the focusing electrode is chosen such that emitted electrons emanating from the emission surface are repelled in a desirable fashion.
Dispenser type thermionic cathodes are a category of thermionic cathodes comprising measures for continuously replacing evaporated material. For example, a dispenser type thermionic cathode may comprise a cathode body with an internal reservoir filled with a material that, upon heating, cause work function lowering particles to diffuse from the reservoir to the emission surface. The presence of work function lowering particles at the emission surface lowers the minimum energy required for electron emission. Unfortunately, work function lowering particles diffused within a thermionic cathode may not only stimulate electron emission, but the particles, or reaction products formed from the particles, may deposit on surfaces of the focusing electrode. Deposition will for example occur when the work function lowering particles are positively charged Barium ions, while the focusing electrode is held at a negative potential for repelling the emitted electrons into an electron beam. Accumulation of work function lowering particles on the surface of the focusing electrode results in dimensional changes and possibly charging of the focusing electrode, which may significantly perturb the electric field applied for focusing the electrons. If the focusing electrode is located close to the cathode emission surface, any particle accumulation on the focusing electrode may also distort the emission distribution at the cathode emission surface. Furthermore, accumulation of particles on the focusing electrode may change its work function, which may lead to increased electron emission from the focusing electrode. These effects may have a negative impact on the quality of the generated electron beam.
In applications using thermionic cathodes, such as electron guns for electron beam lithography, a high and stable electron emission and current density from the cathode may be necessary. In order to achieve this, the alignment of the emission surface with respect to the focusing electrode is critical, as already a small misalignment may cause unacceptable changes in beam properties, such as beam current and/or current density.